This invention relates in general to a system for delineating defects in the surface structure of a crystalline material. The delineated defects can then be used as a measure of crystalline perfection of the material. In a more particular aspect, this invention concerns itself with a method for statically etching the &lt;111&gt; face of an indium antimonide crystal and to an etchant solution for use therewith.
The increased use of infrared sensors for a variety of military and industrial applications has generated a considerable research effort in the development of integrated focal plane arrays for use in the infrared sensors. The use of these arrays provides a dual advantage in that they not only improve sensor performance efficiencies, but, in addition, reduce weight volume and power requirements for associated subsystems. One type of focal plane array which has achieved optimum utilization are the charge coupled infrared imaging devices (CCIRID) which combine monolithic detector elements and charge transfer devices for use in multiplexing or other signal processing functions in a common infrared semiconductor material. Other applications of monolithic infrared CCD's include their use as sensors for earth resource management techniques in agriculture, geology, and coastal zone management; IR astronomy; and meteorological observations from space. Accordingly, the development and generation of new concepts in monolithic infrared imaging has become an important research objective.
The semiconductor material found to be most effective in attempts at accomplishing the research objective referred to above is indium antimonide (InSb). This material is especially useful for CCIRID devices with a photon detection capability in the one to 5.5 .mu.m spectral region. The InSb crystals, however, must be free, or substantially free from surface defects in order to perform at maximum efficiency. A number of methods have been suggested and relied upon in attempting to provide InSb crystals substantially free from crystal lattice defects on the &lt;111&gt; face of the crystal. For example, InSb crystals can be grown by conventional liquid phase epitaxy (LPE), techniques, or by resorting to the well known Czochralski crystal pulling technique (CZ). Significant differences in MOS storage times and leakage currents are often observed in implanted p-n junctions when fabricated on CZ grown InSb wafers as opposed to LPE grown crystals, with both characteristics being found superior for the LPE layers. The electrical difference seems to be correlated, at least in theory, with surface defects observed in the CZ grown InSb wafers but that are totally absent from the LPE grown InSb layers.
As a consequence, the development of an etchant solution capable of detecting defects in the surface structure of an InSb semiconductor material became of paramount importance to the research program's efforts to accomplish its objectives. The purpose of the etchant, of course, is to delineate any defects on the &lt;111&gt; surface of the InSb semiconductor material. The number of defects per unit area provides a measure of crystalline perfection of the semiconductor and an indication of its quality and potential for utilization in the monolithic infrared CCD linear imaging arrays presently being used for a variety of military and industrial applications. Obviously, it is advantageous to select the least defective material in order to ensure that the solid state devices using InSb crystals operate with the highest degree of efficiency.
The preparation of InSb substrates for device fabrication involves polishing processes which, if not carefully controlled, may introduce damage. This damage propagates into the bulk of the substrate to some depth and causes defects which must be removed by chemical polishing prior to device fabrication.
Defect densities are often characterized by the use of etches which delineate crystal lattice defects. Many defect etches exist which delineate crystal lattice defects on the &lt;111&gt; (metal rich) face of InSb substrates. However, the &lt;111&gt; (semi metal rich) face is the one commonly used for device fabrication. Prior to this disclosure, the defect density for a prospective device quality substrate was determined for the &lt;111&gt; face and assumed to be the same for the opposite &lt;111&gt; face. The etchant of this invention however allows for the defect density of a substrate to be determined for the surface on which devices are fabricated, thus eliminating the ambiguity of the previous method.
With the present invention, it has been found that the research objectives referred to above, as well as the facile selection of crystalline material relatively free of surface defects, can be accomplished by using an iodine solution of N,N-dimethylacetamide to statically etch the surfaces of the crystalline material. Etching the crystal surface results in the formation of triangular shaped etch pits that are due to crystal lattice defects. A microscopic examination of the etched surface can then be conducted in order to determine which crystalline material is least defective in order to ensure the best performance for solid state devices using such crystals.